The invention relates to a method for magnetic resonance imaging of a moving object in an examination zone, wherein the nuclear magnetization is excited during an MR examination in the presence of a steady magnetic field by sequences which include at least a respective RF pulse of defined frequency. A number of MR signals received with a defined phase position and produced under the influence of additional magnetic gradient fields are evaluated. A motion parameter of the object is continuously measured for motion compensation and the parameters of the sequence are varied in dependence on or based on the measurement. The invention also relates to an MR apparatus for carrying out the method as well as to a computer program for controlling the control unit of an MR apparatus such that the method can be executed.
Moving objects must be imaged in particular in the field of medical imaging, for example, the heart or the coronary vessels. In this respect, it is known from a study by Wang et al. (MRM 33:713-719 (1995)) that the displacement of the human heart is proportional to the respiration-induced diaphragm motion. Therefore, the cardiac motion can be at least partly compensated for by monitoring the motion of the diaphragm, for example, by means of so-called navigator pulses and by varying the frequency or the phase of the RF pulse or the phase position of the MR signals in dependence thereon, i.e., based thereon (so-called slice tracking), thus reducing the motion artefacts in the MR image.
In another embodiment of the invention which offers even more accurate motion compensation, at least two motion parameters for a motion direction are derived from the measured motion parameters and motion is additionally compensated for by varying the magnetic gradient fields. This provides more accurate motion compensation because when at least two motion parameters are measured, motions that are more complex than a simple translation can be compensated. For example, it is possible to take into account the fact that the heart is compressed or expanded under the influence of the diaphragm motion. Such compression or expansion, but also rotation, however, can be compensated only by varying the magnetic gradient fields acting during a sequence in addition to the frequency and the phase position of the RF pulse or the MR signal.
In order to mitigate this drawback, a publication by Taylor et al in ISMRM, 322 (1998) proposes to derive individual correction factors from MR images that are acquired prior to the actual MR examination, and show the heart in different respiratory phases. This enables more accurate compensation, but the acquisition of the individual correction factors necessitates careful evaluation of the previously formed MR images by the examiner.
It is the object of the present invention to provide a method for imaging a moving object in which a comparatively accurate motion compensation is achieved in a simple manner. This object is achieved in accordance with the invention by performing the following steps:
measuring the variation in time of a number of correlated motion parameters during a preparation phase preceding the MR examination,
measuring a portion of the correlated motion parameters during the MR examination,
deriving the motion parameters that have not been measured during the MR examination from the measured motion parameters, and
varying the parameters of the sequence based on the calculated or measured motion parameters in order to achieve motion compensation.
In conformity with the invention the individual adaptation takes place during a preparation phase that precedes the actual MR examination n correlated motion parameters are then measured quasi simultaneously. The correlation between the individual motion parameters can be derived from such a measurement. During the subsequent MR examination it is merely necessary to measure m motion parameters (for example, the diaphragm motion, m=1) wherefrom the other motion parameters can be derived on the basis of the previously determined correlation. The parameters of the sequence (for example, frequency or phase position of the RF pulse or the MR signal) can be varied in dependence thereon in such a manner that motion compensation is achieved.
The method in accordance with the invention is executed automatically and does not require evaluation of previously formed MR images by an examiner. Even though a large number (n) of motion parameters must be measured for the exact motion compensation, the actual MR examination is not affected thereby, because the acquisition of such motion parameters takes place during the preceding preparation phase.
In another embodiment of the invention which offers even more accurate motion compensation, at least two motion parameters for a motion direction are derived from the measured motion parameters and motion is additionally compensated for by varying the magnetic gradient fields. This provides more accurate motion compensation because when at least two motion parameters are measured, motions that are more complex than a simple translation can be compensated. For example, it is possible to take into account the fact that the heart is compressed or expanded under the influence of the diaphragm motion. Such compression or expansion, but also rotation, however, can be compensated only by varying the magnetic gradient fields acting during a sequence in addition to the frequency and the phase position of the RF pulse or the MR signal.
The version of the method that is disclosed in claim 2 offers even more accurate motion compensation. The reason is that with at least two motion parameters are measure, motions that are more complex than a simple translation can be compensated. For example, it is thus possible to take into account the fact tat the heart is compressed or expanded under the influence of the diaphragm motion. Such compression or expansion, but also rotation, however, can be compensated only by varying, in addition to the frequency and the phase position of the RF pulse or the MR signal, the magnetic gradient fields acting during a sequence.
For the measurement of the motion parameters, navigator pulses may be used to enable the excitation of a volume that is limited in two dimensions, for example a cylindrical rod (pencil beam). For example, when such a navigator pulse is incident on the diaphragm of a patient, the state of motion thereof, or the patient""s respiration, can be determined by evaluation of the MR signals received after the navigator pulse. The motion of a spatially limited volume can thus be accurately measured.
In connection with the imaging of the heart (or the coronary vessels), the sequences may be generated under the control of an ECG. The heart moves not only under the influence of respiratory motion, but also because of the cardiac action, a cardiac cycle being significantly shorter than a respiratory cycle. During the late diastole of the cardiac action (briefly before the R wave in the ECG), however, there is a low-motion phase of approximately 100 ins. When the sequences are produced exclusively during such a low motion phase, the cardiac cycle will not introduce any additional motion artefacts.
The time necessary for the acquisition of all required MR signals can be reduced by generating a number of successive sequences per cardiac cycle after measurement of the motion parameters. To this end, it is necessary to utilize sequences whose duration is shorter than that of the low-motion phase of the heart. However, because this phase is short in comparison with a respiratory cycle, it suffices to measure the motion parameters only once during the relevant cardiac cycle.
An MR apparatus in accordance with the invention includes a magnet for generating a uniform, steady magnetic field, an RF transmitter for generating magnetic RF pulses, a receiver for receiving MR signals, a generator for generating gradient magnetic fields with gradients that vary differently in time and in space, an evaluation unit for processing the received MR signals, and a control unit arranged to control the RF transmitter, the receiver, the generator and the evaluation unit. The control unit is programmed to measure a variation in time of a plurality of correlated motion parameters during a preparation phase preceding an examination phase, measure only a portion of the correlated motion parameters during the examination phase, derive a remaining portion of the motion parameters which are not measured during the examination phase from the measured motion parameters and vary at least one parameter of a sequential generation of a magnetic field during the examination phase based on the derived or measured motion parameters in order to achieve motion compensation. A computer program for a control unit controlling the execution of the method in such an MR apparatus includes means for measuring the variation in time of a plurality of correlated motion parameters during a preparation phase preceding an examination phase, means for measuring only a portion of the correlated motion parameters during the examination phase, means for deriving the motion parameters which are not measured during the examination phase from the measured motion parameters, and means for varying at least one paramerer of a sequential generation of a magnetic field during an examination phase based on the derived or measured motion parameters in order to achieve motion compensation.